The present invention relates generally to methods and systems for radiocommunications and, more particularly, to such systems in which a connection can be handed over from one channel or base station to another.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry's growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices.
In cellular systems, the capability is typically provided to transfer handling of a connection between, for example, a mobile station and a base station to another base station, as the mobile station changes its position and so moves out of the coverage area of one base station and into the coverage area of another base station. This type of handoff is commonly referred to as an "intercell" handoff as the coverage areas associated with base stations are commonly referred to as "cells". Depending upon the quality of the current channel, it may also be desirable to transfer a connection from one channel of the base station to another channel supported by the same base station, which handoffs are commonly referred to as "intracell" handoffs.
To smoothly complete a handoff, the network controlling the base stations first determines, for each mobile station, whether the need for handoff is imminent and secondly determines to which new base station (or channel) handoff should be effected. In making the latter decision it is desirable that the network controller know either how well each base station can receive signals from a mobile station in question, how well the mobile station in question can receive signals from each base station, or both.
Many existing radiocommunication systems are based on an access method known as Frequency Division Multiple Access (FDMA), in which each mobile station transmits on a unique frequency within its current base station area. The mobile station is thus unaware of signals on other frequencies from surrounding base stations. In FDMA systems it is typically considered too costly to equip mobile stations with an extra receiver that could be used to scan other base station frequencies. Instead, it is common for base stations to be equipped with a scanning receiver that searches for the signals of approaching mobile stations. The network then hands over a mobile station from a base station covering an area that the mobile is leaving to the base station that reports the best reception of the mobile station's signal.
More recent cellular telephone standards employ Time Division Multiple Access (TDMA) in which a fixed time period (e.g., 20 mS) on each pair of radio frequency links (one used in the direction from mobile station to base station (uplink), the other used in the direction from base station to mobile station (downlink)) is divided into a number (e.g., 3) of short timeslots (e.g., 6.6 mS) that are cyclically used by different mobile stations. Thus, a first mobile station transmits in the first timeslot in each period, a second mobile station transmits in the second timeslot in each period and so on. Likewise the base station transmits to one mobile station in the first timeslot, another mobile station in the second slot and so on. By offsetting the allocation of timeslots in the two communication directions, base to mobile (the downlink) and mobile to base (the uplink), it can be arranged that a first mobile transmits in the first timeslot and receives in the second timeslot; a second mobile transmits in the second timeslot and receives in the third, while a third mobile transmits in the third timeslot and receives in the first timeslot. An advantage of this arrangement is that a mobile station does not need to transmit and receive simultaneously, which reduces disturbance.
In the above three-timeslot example, each mobile station is active to transmit or receive in two of the three timeslots and is idle in the remaining timeslot. Therefore it is possible for TDMA mobile stations to use this idle time to receive signals from other base stations and measure their signal strength. By reporting these signal strength measurements to the base station using a slow speed data channel multiplexed with the traffic (i.e., voice), the network is informed about the base stations each mobile station can receive. The network can use this information to effect handoff to the best base station, and such a method is termed mobile assisted handover (MAHO). When the base stations scan for the signal strength of mobile stations, the method could be termed base assisted handover (BAHO). Systems providing MAHO also have access to the base station measurements, and so are able to effect smoother and more reliable handovers because both uplink and downlink signal strengths are taken into account, instead of just uplink strengths in the case of BAHO.
In yet another access technique, Code Division Multiple Access (CDMA), mobile stations can share the same frequency band but communications are distinguishable by virtue of unique spreading codes. Even in CDMA systems it is possible to measure a signal strength of pilot channels associated with a particular base station. The base station and/or mobile station can use this information to determine when a handover to another code, or another frequency band in multicarrier CDMA, is desirable.
As can be seen from the foregoing, a common element in determining when an handoff is desirable is the measurement of signal quality received either at the base station, at the mobile station, or both. An intracell handoff, on the other hand, is triggered by insufficient quality of the current connection as indicated by, e.g., bit error rate (BER). Conventionally, the decision to make a handoff is based on this signal quality information in view of some constant hysteresis value. Signal quality can be expressed as either, for example, signal strength or BER. If, for example, the signal strength received by a mobile station from an adjacent cell's base station exceeds that of the base station to which it is currently connected plus the constant hysteresis value, then a handoff occurs to the adjacent cell's base station. By providing the hysteresis value, it was conventionally thought that inadvertent handoffs due to fluctuations in signal strength could be minimized so that a mobile station operating near cell borders didn't experience a "ping-pong" effect between base stations.
However, the provision of a constant hysteresis has been found by Applicants to be ineffective under certain circumstances. In particular, base stations filter the reported signal quality in order to more accurately determine when a handoff is desirable for a particular mobile station under the particular rf conditions that its channel is experiencing. This filter can be initialized either by assuming a fixed value or, for example, by using the first received value from the mobile station. If the first received value from the mobile station is used as a "best guess" to initialize the filter, then the filter will more rapidly converge on a correct value. However, during the convergence process, fluctuations in the signal quality may result in undesirable handoffs.
Accordingly, there is a need to develop enhanced techniques to determine when a handoff is appropriate to efficiently utilize system resources, to avoid oscillating connections caused by the "ping-pong" effect described above and to address the undesirable handoffs caused during the period when receive filters are being initialized.